1. Field of the Invention
This invention relates to an image-forming method making use of developers for full colors, used to carry out recording processes utilizing electrophotography or electrostatic recording. More particularly, it relates to a full-color image-forming method and a two-component developer kit which are used in image-forming apparatus usable in copying machines, printers, facsimile machines, plotters and so forth.
2. Related Background Art
A number of methods are conventionally known as methods for electrophotography. Copies or prints are commonly obtained by forming an electrostatic latent image on an electrostatic-image-bearing member by utilizing a photoconductive material and by various means, subsequently developing the electrostatic latent image by the use of a toner to form a toner image, transferring the toner image to a transfer medium such as paper as occasion calls, and thereafter fixing the toner image to the transfer medium by heat, pressure or heat-and-pressure.
In full-color copying machines, full-color printers and so forth, it has been common to use a method in which a transfer material is wound on the surface of a transfer member holding member (transfer drum) set opposingly to one photosensitive member, the transfer material being wound by an electrostatic force or a mechanical action of a gripper or the like, and the process from development to transfer is carried out four times to obtain a full-color image (a transfer drum system); or a method in which, using four electrostatic-image-bearing members, electrostatic latent images formed respectively on the electrostatic-image-bearing members are developed by the use of a yellow developer, a magenta developer, a cyan developer and a black developer, and, transporting a transfer material by means of a beltlike transfer member, the toner images of the respective colors are transferred to the transfer material, followed by fixing to form a full-color image (a tandem system).
In recent years, as to transfer materials for full-color image formation, it has become increasingly necessary to deal with not only sheets of paper usually used and films for overhead projectors (OHP), but also sheets of cardboard or small-sized sheets of paper such as cards and post cards. In the transfer drum system, since the transfer material is wound on the surface of the transfer drum, there is a disadvantage that cardboard such as post cards cannot be used. On the other hand, in the tandem system, since the transfer material is transported in the state of a flat sheet, this system is widely applicable to various types of transfer materials. However, a plurality of toner images must exactly be transferred without any color differences. Hence, this system tends to cause defective images due to faulty transfer of toner. In order to solve this problem, it has been attempted to add to toners various additives such as a fluidity improver in order to improve toner transfer performance without any color differences to make images high-quality.
However, it is difficult to remove a problem of toner scatter or to satisfy image characteristics such as image density, image quality and so forth of images to be obtained, if the transfer performance and chargeability of the respective color toners are balanced without taking account of the electrical resistance of colorants added internally to toner particles, i.e., the electrical resistance of high-resistance organic colorants and that of inorganic colorants including low-resistance carbon black. Also, if any differences in electrical resistance of colorants are taken into account, color differences may appear in glossiness, image density, coloring power and so forth of the respective color toners, and it is still difficult to satisfy the image characteristics in the case when, as far as color non-uniformity, color balance or the like is taken into account. In Japanese Patent Application Laid-open No. 6-11901, it is attempted to improve transfer performance by making a fluidity improver (titanium oxide) of a black toner have a higher volume resistivity than color toners, taking account of the differences in electrical resistance of colorants. This method certainly brings about an improvement in transfer performance, but may take the desired effect with difficulty when the titanium oxide is in a small quantity, or, when it is added in a large quantity, the black toner tends to have a low chargeability, and hence the fluidity improver cannot be used in the required quantity from the viewpoint of improvement in fluidity. Thus, it has been difficult to achieve both toner fluidity and transfer performance.
Meanwhile, image-forming methods making use of an intermediate transfer member (intermediate transfer system) are also proposed.
For example, a full-color image forming apparatus making use of a drum-shaped intermediate transfer member is disclosed in U.S. Pat. No. 5,187,526. Also, in Japanese Patent Publication No. 7-50358, a recording method is disclosed in which a plurality of color toner images are superimposingly transferred in sequence from a plurality of electrostatic-image-bearing members to an intermediate transfer member, and the toner images on the intermediate transfer member are further transferred to a transfer material. The intermediate transfer system has advantages in that it makes any complicated optical system unnecessary, it can also be used to form images on stiff paper such as postcards and cardboard, and still also it can make the apparatus itself compact when an intermediate transfer belt is used because the belt is flexible.
In the system making use of such an intermediate transfer member, the toner image is primarily transferred from the electrostatic-image-bearing member such as a photosensitive member to the intermediate transfer member and thereafter it must further be transferred from the intermediate transfer member to the transfer material. Accordingly, the toner transfer efficiency must be made higher than that in conventional cases. More specifically, in developing systems making use of the intermediate transfer member, the toner image is transferred twice so as to be formed on the transfer material, and hence any image defects concerning the transfer performance of toner tend to occur.
In a full-color intermediate transfer system making use of the intermediate transfer member, toners are present on the intermediate transfer member in a larger quantity than in the case of a monochromatic black toner used in black-and-white copying machines, and hence their transfer efficiency more tends to lower. More specifically, in the formation of a full-color image, toner images formed by superimposing two or more color toners may uniformly be transferred with difficulty, so that, when the intermediate transfer member is used, problems tend to occur in respect of color uniformity or color balance. Thus, it is not easy to stably reproduce full-color images having a high image quality.
In particular, with toner images formed by superimposing two or more color toners inclusive of a black toner, a poor transfer performance may cooperatively result because of a lower transfer performance of the black toner to tend to cause image defects. A black toner making use of an inorganic colorant including low-resistance carbon black as a colorant may have a lower charge quantity than a yellow toner, a magenta toner and a cyan toner (hereinafter often simply “color toners” distinguishing from the black toner) each making use of a high-resistance organic colorant, and also tends to cause a lowering of transfer efficiency with a decrease in charge quantity at the time of its transfer from the electrostatic-image-bearing member to the intermediate transfer member and a lowering of transfer efficiency with a decrease in charge quantity at the time of its transfer from the intermediate transfer member to the transfer material, as well as line-image blank areas caused by poor transfer, fog, and toner scatter (black spots around line images). Also, its transfer performance and chargeability may greatly be influenced depending on the state of dispersion of carbon black in toner particles. Hence, the above problems may become remarkable especially in the image-forming method making use of the intermediate transfer member, in which the toner image is transferred twice. Accordingly, it is difficult for the black toner to be improved in its balance with the color toners.
In order to remove the transfer residual toner coming at the time of transfer from the intermediate transfer member to the transfer material, the intermediate transfer member is also often provided with a cleaning member, which, however, is not preferable in view of the lifetime of the intermediate transfer member. Accordingly, it is sought to improve the transfer efficiency.
Moreover, black-and-white images are reproduced in an unexpectedly large proportion even in full-color printers. Hence, the black toner or developer may deteriorate earlier than the color toners, so that the difference in transfer performance may become more pronounced between the black toner and the color toners to cause image defects.
Meanwhile, in order to prevent developers from deterioration to achieve stable developing performance even in long-term use, a developing assembly of a system in which the carrier in a developer chamber is little by little supplied has been proposed. As a typical example of the developing assembly of such a system, as disclosed in, e.g., Japanese Patent Publications No. 2-21591 and No. 1-43301 and Japanese Patent Application Laid-open No. 3-145678, a developing assembly is known in which a virgin carrier is little by little supplied from a developer supply unit to the developing assembly, and on the other hand the old developer remaining in the developing assembly to become excess as a result of this supply is discharged by making it overflow from a developer-discarding opening to collect it in a developer collection container.
In such a developing assembly, the chargeability of the developer continues deteriorating until a certain service time has lapsed after the virgin carrier has been filled in the developing assembly, but thereafter becomes stable to become substantially constant as the virgin carrier is little by little supplied and the old developer is little by little discharged. Also, the developer collection container may be replaced after it has become full with the developer collected, and hence it is unnecessary to make any periodic replacement in accordance with the deterioration of the carrier. Also, such a developing system, though having a complicated main-body construction, can be a system having an advantage that any toner may fly up with difficulty when only the developer collection container is replaced. Moreover, any periodic replacement with fresh developers by a serviceman or someone else may be made less times, making it possible to reduce running cost.
However, in a developing system having a carrier supply unit in addition to a toner supply unit as disclosed in Japanese Patent Publication No. 2-21591, the carrier and the toner must be supplied in quantities controlled independently. This not only makes their control complicated, but also cannot make the apparatus compact. Also, as disclosed in the above Japanese Patent Publication No. 1-43301 and Japanese Patent Application Laid-open No. 3-145678, in a developing system in which the carrier is incorporated in a replenishing developer, a stable chargeability of the developer can certainly be maintained over a long period of time. However, the problems in transfer performance as discussed above must be eliminated before the demand in recent years for making image quality higher is satisfied.